Letters written with a solution of acetate of lead, are invisible until exposed to the action of sulphuretted hydrogen, which makes them distinct, with the lustrous greyish black of sulphuret of lead, the same substance which is called galena when it occurs as lead-ore. A weak infusion of galls or other vegetable astringent, will, if applied to paper in the form of letters, become legible when touched with any solution of iron. If written with a solution of ferro-cyanide of potash, letters will remain invisible until touched with a solution of sulphate of iron. IMPORTANCE OF GOOD INK. Astle speaks very impressively and justly on this point; and we contribute to this part of our subject by calling attention to facts almost daily occurring or brought to notice in this country, especially in the older cities and states, where town-records, parish-registers, and other documents of ancient date, and of high importance in history, chronology, and genealogy, (as well as in regard to the title and inheritance of estates,) are found obscured and obliterated, causing losses, public and private, that need but to be mentioned to be properly estimated. In the appendix will be found a fac-simile of a sheet upon which various specimens of ink were thoroughly and fairly tested, which is a brief but emphatic demonstration of a difference of qualities by difference of results. To show what can be done in the preservation of writing on material even frailer than such paper as we employ, we need but produce the specimen of Egyptian writing on papyrus, pronounced by Champollion to have been executed more than sixteen hundred (1600) years before the birth of Christ, yet still in preservation and legible, as may be seen by the representation we give of it. This is undoubtedly as old as any specimen of phonetic characters or written letters (representing sounds, not ideas or objects,) extant, made by marking with a fluid upon any substance. There are inscriptions of letters upon stone, for which an earlier date of 4000 years B.C., is claimed with truth. But this is INK-writing, absolutely 3500 years old!